1. Field of the Invention
The present invention relates to a lift cylinder (a hydraulic cylinder) in a fork lift, and in particular to a technology for detecting a position of a piston in the lift cylinder.
2. Description of the Related Art
In general, a lift cylinder (a hydraulic cylinder) is installed in a fork lift to move a fork upwardly and downwardly. By reciprocating a piston rod in the lift cylinder, the fork is moved upwardly and downwardly. Hence, in a case where various controls are applied to the fork in the fork lift having the lift cylinder, the upwardly and downwardly moved positions and upwardly and downwardly moving speeds of the fork are detected by measuring a position and a moving speed of the piston rod (the piston) of the lift cylinder.
As means for detecting the position or the moving speed of the piston of the lift cylinder, an ultrasonic wave sensor is typically used. An example of the lift cylinder using such ultrasonic wave sensor is described in Japanese Patent Laid-Open No. 10-238513.
In the lift cylinder described in this publication, an ultrasonic wave sensor for transmitting and receiving an ultrasonic wave is installed at a position where it is confronted with a bottom surface of a piston reciprocating within the lift cylinder (using hydraulic oil as a power medium). The ultrasonic wave transmitted from the ultrasonic wave sensor toward the piston (i.e., a transmitted wave) is transmitted through the hydraulic oil and reflected by the bottom surface of the piston, so that the reflected ultrasonic wave (i.e., a received wave) is received by the ultrasonic wave sensor. Since the speed (i.e., the sonic speed) of the ultrasonic wave transmitted through the hydraulic oil is affected by the temperature, a temperature sensor is set within the lift cylinder so as to detect the temperature to correct the sonic speed. The data of the sonic speed thus corrected depending on the temperature within the lift cylinder and the data on a time lag (a measurement time length) of the received wave relative to the transmitted wave are subjected to an arithmetic processing in a control device, thereby calculating a distance (a measurement length of distance) between the ultrasonic sensor and the bottom surface of the piston. With this measurement length of distance, the position and the moving speed of the piston within the lift cylinder is structured to be detected.
However, it is generally known that the speed of the ultrasonic wave transmitted through the medium (for example, the hydraulic oil) is affected by not only the temperature but also the pressure. For example, under a condition in which the temperature is constant, if the pressure within the cylinder is increased, the sonic speed transmitted through the hydraulic oil is increased, and therefore the estimated measurement length distance thus calculated and detected becomes shorter than the accurate measurement length distance between the ultrasonic wave sensor and the bottom surface of the piston.
Consequently, in the above-mentioned conventional position detecting device using the ultrasonic wave sensor as a detecting means, it is impossible to obtain the measurement length distance (the position of the piston with respect to the ultrasonic sensor) with high accuracy if the pressure within the cylinder is varied, and thus it was difficult to detect the position of the piston within the cylinder accurately.
Accordingly, an object of the present invention is to provide a position detecting device for a hydraulic cylinder and a position detecting method thereof, which can accurately detect a position of a piston within the cylinder using an ultrasonic wave sensor even if the pressure within the hydraulic cylinder is varied.
A position detecting device for a hydraulic cylinder according to the present invention is a hydraulic cylinder position detecting device for detecting a position of a piston with a ultrasonic wave, the piston being movable within a cylinder, the detecting device including an ultrasonic wave sensor provided within the cylinder for transmitting the ultrasonic wave toward the piston and receiving the ultrasonic wave reflected by the piston, a pressure sensor for detecting a pressure of fluid through which the ultrasonic wave is transmitted within the cylinder, and arithmetic calculation means for calculating a sonic speed of the ultrasonic wave based on the pressure of fluid detected by this pressure sensor to calculate the position of the piston based on this sonic speed.
A position detecting method for a hydraulic cylinder according to the present invention is a hydraulic cylinder position detecting method for detecting a position of a piston with an ultrasonic wave, the piston being movable within the cylinder, the detecting method including the steps of transmitting the ultrasonic wave toward the piston and receiving the ultrasonic wave reflected by the piston, detecting a pressure of fluid through which the ultrasonic wave is transmitted within the cylinder, calculating a sonic speed of the ultrasonic wave based on the detected pressure of fluid to calculate the position of the piston based on this sonic speed.